1. Field of the Invention
The invention relates to a method for optically monitoring fabrication processes of finely structured surfaces in a semiconductor fabrication and a device for carrying out the method. A comparison with existing diffraction images of tested structures is carried out in order to analyze diffraction images of the surface to be examined.
Specifically when fabricating semiconductors, line widths and profiles of structured layers must often be monitored during the fabrication process. Exact compliance with the specifications for the line width is of decisive importance for the functional capability of the product. In addition, further structural parameters such as trench depth or a lateral incline, for example, are of great significance. Suitable measuring devices are necessary to monitor these fabrication parameters on lithographic masks, on semiconductor wafers or on finely structured surfaces.
With the extremely small structural widths in the range of 0.25 μm which are used today, it is no longer possible to use conventional, nondestructive optical line width measuring devices due to diffraction and interference effects. So that as few monitor wafers as possible are required, cost-effective measuring methods for nondestructive and noncontaminating testing of line structures on product wafers are required in semiconductor fabrication. The measuring speed should be such that, for example, after a critical process step, each product wafer can be monitored without significantly increasing the process time.
Electron microscopes which require complex handling and have a low throughput rate are presently used for measuring line widths of fine structures (<1 μm) so that only a small proportion of the processed semiconductor wafers can be tested. Furthermore, precise measurement results for the line profiles are obtained only with what is referred to as cross section recordings, for which an already processed semiconductor wafer has to be destroyed. In addition to the regular product wafers, so-called monitor wafers are therefore also processed during the semiconductor fabrication and are subsequently used for measurement purposes. Especially with future large wafer diameters of 300 mm and above, these monitor wafers will give rise to high costs, firstly due to the material value itself, and because they significantly reduce the throughput rate of product wafers.
One approach for a solution to this problem is provided by the diffraction measurement or scattered light measuring method, referred to as scatterometry. In general, in this method the measuring region which is to be examined is illuminated and conclusions are drawn about the surface properties of the measuring region from the features of the reflected light. If there are periodic structures on the substrate, given a corresponding selection of the light wavelength, diffraction and interference effects occur. These diffraction and interference effects prevent a measurement with the usual optical devices. However, the scattered light measurement and diffraction measurement register and evaluate the diffraction and interference effects because they are characteristic for the structural variables of a measured surface. Using complex model calculations it is already possible to determine various structural variables such as line width, edge incline or line height through the use of scattered light measurement.
The reflection of coherent light at periodic structures which can be understood as amplitude or phase lattices results in diffraction effects and interference effects. If the wavelength of the light used is at least larger than half the lattice period, further maximum diffraction values of a higher order are produced in addition to the directly reflected beam of the zero-th order. The position or angle θn of the n-th order of diffraction depends only on the angle θi of incidence, on the lattice period g and on the wavelength λ:             sin      ⁢                          ⁢              θ        i              +          sin      ⁢                          ⁢              θ        n              =      n    ⁢                  ⁢          λ      g      
In the case of two-dimensional lattices and complicated structures with a plurality of different periods, the diffraction problem has to be analyzed in a three-dimensional fashion. If the size of the examined structures lies in the range of the wavelength, the simple Fraunhofer diffraction equations no longer apply. Instead, the Maxwell equations for reflection and transmission in lattices must be explicitly solved, for example using what is referred to as rigorous coupled wave analysis. The nonlinearities which occur permit generally valid statements to be made only in a very limited fashion, for this reason when assessing diffraction effects on small structures it is always necessary to consider the specific individual case or to make numerical calculations.
The intensities and the phases of the orders of diffraction depend here on the properties of the incident light (angle, polarization, wavelength), on the examined lattice structure (lattice periods, line width, line height, layered structure, degree of rounding of edges, roughness) and on the material properties of the substrate (refractive index, absorption index). However, the position of the maximum diffraction values is influenced only by the angle of incidence, the lattice period and the wavelength. If these variables are constant, it is possible to draw conclusions about the other lattice parameters from the intensity evaluation of the locally fixed orders of diffraction. Because of the large number of lattice influencing variables, the lattice parameters can be determined unambiguously only if a sufficient number of measured intensity values is available for the examined measuring point.
The determination of lattice parameters by comparing the measured diffraction images with reference diffraction images which have been calculated from the circuit layout with an enormous calculation effort has, at the experimental stage, not yet been satisfactory, namely only for the case of exclusively parallel lines. Measuring devices for measuring the diffraction images according to the prior art are disclosed, for example, in Published German Patent Application No. DE 198 24 624 and U.S. Pat. No. 5,703,692. These devices are used at great cost for determining lattice parameters on strictly periodic structures when manufacturing DRAMs (Dynamic Random Access Memory) by using the following measures. In addition to the DRAM circuits, a geometrically simple test structure of parallel strips is applied to a wafer. Only its diffraction image is then measured and compared with slightly varied reference spectra of the geometrically simple test structure. The lattice parameters of the test structure result from this comparison. The parameters are used to draw conclusions about the lattice parameters of the DRAM circuits. This conclusion may not account for, for example, systematic errors of the lithographic machine or uneven plasma when generating layers or a grain of dust under the wafer.